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. This pulling force is exerted on its attachments, If the pulling force of the muscle’s contraction is If the pulling force of the muscle’s MUSCLE BIOMECHANICS A typical muscle attaches from the bone of one body part to the bone of an adjacent body part, thereby (Figure them between located is that joint the crossing 2). The essence of muscle function is that when a muscle contracts, it creates a pulling force toward its center (14) attempting to pull the two body parts toward each oppose the forces that resistance also There are other. movement of each of the body parts. Most commonly, this resistance force is the force of gravity acting on the mass of each body part and is equal to the weight of the body part. will muscle the force, resistance the than greater contract and shorten, termed a concentric contraction, that is crossed at the joint body part will move and the .

(5) “Perhaps no muscles are are no muscles “Perhaps and misunderstood more dysfunction more have the them than attributed to Looking at psoas muscles. that the the multiple joints and ... psoas major crosses, see why. it is easy to . Because . (15)

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Psoas Major Function Psoas by Joseph E. Muscolino | illustrations by Giovanni Rimasti Giovanni by | illustrations Muscolino Joseph E. by Although variations occur for every muscle, including What its attachments are fairly clear. the psoas major, are not entirely clear are the biomechanical effects that especially attachments, its upon has major psoas the major is psoas regard, the in this Indeed, the spine. upon likely the most controversial muscle in the human body. INTRODUCTION The psoas major is a multijoint muscle that spans from Its proximal the thoracolumbar spine to the femur. attachments are the anterolateral bodies of T12-L5 and the discs between, and the anterior surfaces of the transverse processes of L1-L5; its distal attachment is 1) (Figure femur the of trochanter lesser the A Biomechanical Examination of the Psoas Major Psoas of the Examination A Biomechanical the psoas major blends distally with the iliacus to attach these two muscles are often onto the lesser trochanter, described collectively as the iliopsoas. Some sources also include the psoas minor as part of the iliopsoas rt C rt Expe Body Mechanics 18 mtj/massage therapy journal spring 2013 contraction is described asan eccentriccontraction isdescribed contraction. force, themusclewilllengthen outasitcontracts andits the muscle’s contraction force isless thantheresistance asan isometriccontractioncontraction isdescribed . If length ofthemuscledoesnotchange, andthemuscle’s attachments ofthemusclewillnotmove, therefore the contraction force isequalto theresistance force, the termed aconcentricshorten, contraction themuscle’s. If the resistance force, themusclewillcontract and muscle contraction compared to theresistance force. its lengthisdetermined by therelative strength ofthe astrongor perhaps wind. When amusclecontracts, exercise, anotherpersonpushing/pullingonourbody forces otherthangravity are addedweights to an muscles inourbody. resistance Examplesofexternal internal resistance forces are the contractions ofother resistance to Examplesof themovement ofabodypart. forces, bothinternal canalsoprovide andexternal, force becauseitisgenerated outsideofthebody. Other asanexternal isdescribed movement ofabodypart The resistance force that iscreated by gravity to Isometric C C the axisforeachcardinalplane. examine separatelythemuscle’s lineofpullrelativeto the motion/jointactionineachplane,wewouldneedto (i.e., sagittal,frontal,ortransverseplane),todetermine actions aretechnicallymotionswithinacardinalplane (antagonistic) jointaction(Figure4).Giventhat the joint,itwillhaveabilitytocreateopposite joint action;ifitslineofpullpassesontheotherside side ofthejoint,itwillhaveabilitytocreateone by themuscle.Ifamuscle’s lineofpullpassesonone that generallypassesthroughthejointiscrossed of thatjoint.Theaxismotionisanimaginaryline of pull of the muscle relative to the axis of motion The parameterthatneedstobedeterminedistheline muscle’s pullingforcerelativetothejointthatiscrossed. figured outbyanalyzingthebiomechanicsof (Figure 3). approximately equal andboth attachments will move resistance tomotionforeachofthetwobodypartsis of theotherbodypart.However, insomecases,the to movement is less than the resistance to movement of thetwoattachmentsmovesbecauseitsresistance joint actionsthataredescribed.Generally, onlyone in textbooks,itisthemuscle’s concentriccontraction by themuscle.Whenamuscle’s jointactionsarelisted Body Mechanics If themuscle’sIf contraction force isgreater than The jointactionthatamusclecancreatebe oncentric, Eccentric and ontractions The (3 SkeletalMusclesof theHumanBody from permission Muscolino,kind J. E., The MuscularSystem Manual: thereby crossing thejoint located between them.Reproduced with muscleattaches to A typical thebonesoftwo adjacent bodyparts, $ Body (3 Body The MuscularSystem Manual: The SkeletalMusclesoftheHuman drawn from permission in.Reproduced Muscolino, withkind J. E., abdominishasbeenghosteddrawn rectus and in;andtheleft Anterior viewofthepsoasmajormuscles. iliacushasbeen The left Figure 2 Figure 1 rd ed.). Mosby ofElsevier. rd ed.). Mosby ofElsevier. Iliacus major Psoas abdominis Rectus % www.amtamassage.org/mtj 19 ed.). ed.). 0RELOH rd % 0RELOH $ & All of these factors help to explain why the psoas 0RELOH B the hip joint, the spine also allows motion in all three the hip joint, the spine also allows motion psoas major of the examination planes, so our cardinal actions in each must also consider the possible spinal of the three cardinal planes. What further complicates actions is the a clear understanding of the psoas major’s fact that the lumbar spine is not monolithic. There are many joints within the lumbar spine, each with its own axis of motion; therefore, each of these joints must be interposed between And finally, considered separately. the spinal and femoral attachments of the psoas major is the pelvis. Therefore, the pull of the psoas major can affect the posture of the pelvis. Changing the posture of the pelvis can then change the posture of the lumbar vertebrae, which can change the line of pull of the psoas major relative to the axes of motion of the lumbar spinal joints and therefore possibly change the action of the psoas major. major can be so challenging to understand. Following is an examination of the functions of the psoas major )L[HG % . Therefore, $ % (14) )L[HG % ; therefore, to determine its actions, we ; therefore, to determine its actions, we (5, 9, 12) $ However, the psoas major is more complicated because complicated more is major psoas the However, A Figure 4 Figure Figure 3 Figure $ need to compare its line of pull at the hip joint in each of the three cardinal planes. Standard actions at the hip joint are considered to involve movement of the distal attachment—in other words, the thigh. These actions occur when the lower extremity is in what is known as “open-chain” position, with the distal segment, the if the foot is planted on foot, free to move. However, the ground and the lower extremity is in closed-chain position, the pelvis moves at the hip joint instead; when the proximal attachment moves instead of the distal attachment, this is called a reverse action BIOMECHANICS OF THE PSOAS MAJOR BIOMECHANICS OF THE PSOAS MAJOR a muscle of the The psoas major is first and foremost hip joint , Flexion of the thigh at of the thigh at action. its joint determines the axis of the joint A, Flexion to line of pull relative a muscle’s that view showing Right lateral dot. the red by axis is represented The Note: B, Extension the hip joint. the hip joint. of the thigh at , Both attachments “A” and “B” move. move. “B” and “A” C, Bothattachments moves. “B” B, Attachment moves. “A” A, Attachment a muscle. of contractions Concentric Muscles of the Human Body Skeletal (3 The Manual: Muscular System The E., J. with kind Muscolino, Reproduced permission from 0RELOH Mosby of Elsevier. Mosby a thorough examination of the psoas major at the hip joint involves consideration of its standard and reverse actions at that joint. it also crosses the lumbar spine, therefore we need to also examine its line of pull across the spine. As with 20 mtj/massage therapy journal spring 2013 perpendicular angle(see Figureperpendicular 6). joint alongalinethat meetsthemuscle’s lineofpullat a isthemeasurearm ofthedistance from theaxisof the weaker isthemuscle’s contraction force. Amoment contraction force; thecloserlineofpullisto theaxis, and therefore thestronger istheeffectofmuscle’s axis ofmotion,thegreater istheleverage/moment arm, themuscle’s effect,thefarther In lineofpullisfrom the its leverage force, ormoment at arm, thejoint crossed. that determines amuscle’s factor external strength is physiological area cross ofthemuscle. sectional The internal strength iseffectively determined by the pennate inarrangement), themeasure ofamuscle’s this equation (whetherthemuscleispennate ornon- architectural arrangement ofthemusclefibersaffects cross-bridges withinthemuscle.actin the Because sarcomeres, ormore specificallythenumberof myosin- which isessentially determined by thenumberof istheinternalinternal strength factor ofthemuscle, be dividedinto factors. internal andexternal The major will have whencreating thismotion. These can factors looked at to determine thestrength that themuscle joint’s axisofmotion.However, mustbe otherfactors ofthelinepullmusclerelativea factor to the amusclecancreate whatDetermining joint action is C S tilts atthehipjoint(Figure5). If weareinaclosedchainposition,thepelvisanteriorly an open-chainposition,thethighflexesathipjoint. Figure 4A);therefore,itflexesthehipjoint.Ifwearein passing anteriortothemediolateralaxisofmotion(see at thehipjoint.Itclearlycrossesjointanteriorly, controversy overthepotentialactionofpsoasmajor S thigh atthehipjoint. and theclosed-chainmotionsofpelvisrelativeto motions ofthethighrelativetopelvisathipjoint planes. Further, weneedtoconsidertheopen-chain effect of the psoas major in each of the three cardinal three cardinalplanes.Therefore,wecanexaminethe The hipjointisatriaxialthatallowsmotioninall regarding psoasmajorfunction. perhaps resolvemanyofthereasonsforcontroversy in thefieldofkinesiology, andattempt toexplainand psoas majorfunctionbymanyoftheleadingauthors we willconsidersomeofthecompetingassertionsfor at boththehipandspinaljoints.Inourdiscussion, Pso Body Mechanics agittal PlaneInthesagittalplane,thereislittleorno trength ofaMuscle’s ontraction as M a jor Hip Joint Actions major wouldincrease,andtherefore itsstrengthand thigh werefirstinflexion,the momentarmofthepsoas flexion moment armsareatanatomic position. If the it isunipennate architecture ofthepsoasmajorisnotfusiform;rather, at thehipjointisstability. Hepointsoutthatthefiber also believesthattheprimaryroleofpsoasmajor the term“centration”todescribethisidea.SeanGibbons joint byisometricallycontracting contracting, but rathertostabilizethebonesofhip move thebonesathipjointbyconcentrically the primaryroleofpsoasmajorisnottoactually the psoasmajorasa“majormuscleofhipflexion” effusively. JanetTravell and David Simons described on todescribethepsoasmajor’s hipflexionrolerather 5, 9) the acetabulumasotherhipflexorscontract joint istostabilizeandcentertheheadoffemurin believes thatthemajorroleofpsoasathip efficiently pullthehipjointintoflexion.EvanOsar flexor muscleswithgreatermomentarmswouldmore psoas ispurelyasahipflexor.” its efficiencyasahipflexor.” to shortenislessthanbelieved.Thiscallsintoquestion range ofmotion.Therefore,“…theabilitythemuscle nonpennate musclesaredesignedtoproduceagreater produce greaterforceoverashorterdistance,whereas words, themostpowerfulmover)ofhipjointflexion a wholeisoftenstatedtobetheprimemover(inother the psoasmajorversusiliacus,butiliopsoasas cases, itcanbedifficulttodeterminewhatascribe major along with the iliacus as the iliopsoas. In these sources statethathipflexionisitsprimaryfunction psoas majorisaflexorofthehipjoint.Infact,most S “strong” and itshipflexionrolehasbeendescribedbyothersas degrees of flexion of motion,andthatitisstrongestbetween4560 is relativelyweakatthebeginningandendranges One study asserts that the psoasmajor’s hipflexion is asconvincedofthepoweritshipflexionability. major to create flexion at the hip joint, not every source sectional area hip flexor”becauseithasalargephysiologicalcross specifically pointsoutthatthepsoasmajorisa“strong mediolateral axisofmotion(Figure6) because themuscle’s lineofpullpassessoclosetothe than themomentarmformostofotherhipflexors out that the moment arm of the psoas major is smaller agittal Plane: However, itshould benotedthatthesecomparative Therefore itwouldmakesensethattheseotherhip Although nosourceconteststheabilityofpsoas . StuartMcGillgoesasfartostate“Theroleofthe (5) , “powerful” (20) T high Flexion Allsourcesconcurthatthe (2, 31) . Sometimes authors discuss the psoas (31) . In fact, many sources believe that . Pennatemusclesaredesignedto (6) , or “dominant” (2) (12) . . Andmanysourcesgo (2, 21,26) (19, 20) (19) . Theypoint . . Carol Oatis (21) . Heuses (4) (27) . (3, ; www.amtamassage.org/mtj 21 . (14, 19) . This assumes that . This Regarding closed-chain ilt Regarding T . Interestingly, if the thigh is . Interestingly, (14, 19, 25, 29) (14, (27) . Note: This will have important . Note: This will have . When stated, most sources claim . (6) . However, occasional sources claim it occasional sources claim . However, (4, 5, 19) high Abduction action plane frontal The T (12, 19, 29) Flexion of the thigh (8, 21, 25, 27) Extension of the thigh E Regardless of whether the psoas major is in position To understand this debate and determine whether understand To agittal Plane: Pelvic Anterior Anterior Pelvic Plane: agittal C ramifications for the spine when discussing the effects ramifications for the the spine later in this article. of the psoas major upon joint, hip the at plane frontal the Plane Within Frontal of the if the open-chain standard action is abduction reverse action is thigh at the hip joint, the closed-chain (Figure 8) depression of the pelvis at the hip joint S the pelvis at the hip joint, motion of the sagittal plane into would pull the pelvis of the psoas major line of pull joint the hip tilt at anterior that the psoas major is an abductor of the thigh at the of the thigh psoas major is an abductor that the hip joint first laterally rotated (Figure 9C), we see that the lesser line of trochanter moves laterally and the psoas major’s pull also moves lateral to the axis creating/increasing its ability to perform abduction of the thigh at the hip joint. This is an excellent example of a muscle whose action changes depending on the angle of the joint. or adduction, given how small to perform abduction Frontal Plane: Frontal than the of the psoas major may be more controversial as often but is not debated near sagittal plane activity, its weak frontal because it is far less important due to sources plane leverage force. In fact, many prominent Stuart and Neumann Don Anatomy, Gray’s as such the in major psoas the address even not do McGill frontal plane to be an adductor the pelvis is fixed to the trunk as the trunk is pulled the pelvis is fixed in the lower extremity position Closed-chain anteriorly. the foot is planted on the ground. usually occurs when major closed-chain function is For this reason, psoas for standing posture. If the baseline especially important flexor musculature, including the tone of bilateral hip tight, it will create an increased anterior is psoas major, tilt of the pelvis we need to the psoas major is an abductor or adductor, anteroposterior the to relative pull of line its examine 9). (Figure joint hip the at motion plane frontal of axis the pull of line of the (Figure 9A), anatomic position In psoas major may actually pass medial to the axis of psoas major that the it would seem therefore, motion, if the thigh is first abducted However, is an adductor. (Figure 9B), then we see that its line of pull moves to the lateral side of the axis and the psoas major becomes an and Simons state that the psoas In fact, Travell abductor. been has abduction after abduction assists only major initiated by other muscles ed.). ed.). nd ior tilt of the pelvis B Anter Posterior tilt of the pelvis B D xor Hip fle musculature A

Flexion of the thigh Neutral postion What to conclude from this discussion? There is no What to conclude from this discussion? it is true that deeper muscles at a joint Generally, Extension of the thigh Figure 5 Figure Mosby of Elsevier. Mosby potential role in creating flexion motion at the hip joint potential role in creating flexion motion found a study mentioned, previously (as increase would 45 and 60 the psoas major to be strongest between degrees) (Figure 7). to line of pull is anterior doubt that the psoas major’s creates a force the hip joint and that its contraction seems to of flexion at the hip joint. The only question important for be whether this hip flexion force is more do however, motion or for stabilization. These concepts, a muscle can not need to be mutually exclusive because have a stabilization role as well as a role in motion. tend to function more for stabilization than for motion, to it show does location major’s psoas the at looking and given all the other hip flexor be a deep muscle. Further, muscles that exist with greater moment arms, it is likely that they would more efficiently act toward creating hip flexion motion. This all points to the psoas major acting primarily as a stabilizer of the hip joint when we are in anatomic position and/or when lesser hip flexion force But the psoas major is a large and powerful is necessary. muscle and it would make sense that if a greater hip flexion contraction force were needed, then the psoas major would be recruited to assist in the creation of this motion. This is especially true if the hip joint were arm moment increased the of because flexed, already leverage. Flexion at the hip joint. A, Open-chain the hip joint. at at flexion of the thigh Flexion B, Closed-chainthe hip joint. the hip tilt of the pelvis at anterior E., J. with kind Muscolino, Reproduced joint. permission from (2 and Muscle Function Skeletal System The Kinesiology: A E

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